It is necessary to control access of personnel and vehicles to all sites, both civilian and military.
In recent decades, motor vehicles have been used in the type of terrorist activities that have come to be known as car bombings. Motor vehicles have carried explosives and detonated in a particular location to cause damage to personnel and property.
It is also the case that a motor vehicle that can legitimately be brought onto a site can be stolen or hijacked by a terrorist group, loaded with explosives and brought onto a site by a terrorist or brought in unawares by driver/passengers.